As a common method for analyzing samples, an oxidation-reduction reaction is used. An example is a quantitative procedure disclosed in JP-A2001-330581, which makes use of a biosensor that provides a liquid reaction field.
As will be expected from FIG. 6 of the present application, the biosensor disclosed in the gazette makes use of a capillary which is formed by laminating a cover 92 onto a substrate 90 via a spacer 91, for measurement of blood sugar level. The substrate 90 has a surface formed with a working electrode W, a counter electrode C and a liquid junction sensing electrode S. Though not illustrated in the figure, a reagent region is provided so as to bridge at least an end of the working electrode W with an end of the counter electrode C. The reagent region includes an oxidation-reduction enzyme and an electron transfer material.
In this biosensor 9, when blood is introduced into the capillary, a liquid reaction field is formed which includes the oxidation-reduction enzyme, the electron transfer material and glucose, in the capillary. In the liquid reaction field, electron transfer takes place between glucose and the electron transfer material. The electron transfer material becomes a reductant (or an oxidant). Then, when a voltage is applied to the liquid reaction field via the working electrode W and the counter electrode C, electron transfer takes place between the working electrode W and the reductant (oxidant), which generates a response current necessary for analyzing the sample. On the other hand, by applying a voltage to the liquid reaction field via the liquid junction sensing electrode S and the working electrode W (or the counter electrode C), it is possible to obtain a response current necessary for detecting whether or not the capillary has been filled with the sample. The voltage application via the liquid junction sensing electrode S and the working electrode W (or the counter electrode C) ceases if the response current for detection exceeds a predetermined value. Specifically, if a liquid junction is detected between the liquid junction sensing electrode S and the working electrode W (or the counter electrode C), blood has already reached the liquid junction sensing electrode S. So, detection of a liquid junction justifies an assumption that the capillary has been filled with the sample.
However, according to the quantitative procedure described above, the response current for detection is obtained simultaneously with the obtainment of the response current for analysis. In other words, a voltage is applied to the liquid reaction field simultaneously, for a different purpose than obtaining a response current for analysis, using the liquid junction sensing electrode S. During this process, part of glucose which is supposed to be used for the measurement of response current for analysis is used for the detection of sample supply. Further, this causes non-uniformity in the concentration of glucose or reductant (oxidant) in the reaction field. As a result, the response current for analysis does not necessarily reflect the glucose concentration appropriately. Further, severity of non-uniformity is not always the same, which leads to a problem of decreased measuring accuracy.